Electric propulsion system including heaterless dispenser cathode

ABSTRACT

A circuit for igniting and sustaining an electron discharge includes an ignitor circuit. The igniter circuit includes a high voltage transformer and a switch connected in series between a primary of the transformer and a DC source return. The switch is configured to receive a driving signal. A reset circuit is connected in parallel to the primary of the high voltage transformer. A first rectifier is connected in series between a secondary of the high voltage transformer and a keeper. A terminal of the secondary of transformer is connected to a cathode. The circuit for igniting and sustaining the electron discharge also includes a sustaining circuit having a current source with a return connected to a cathode and a second rectifier connected in series between the current source and the keeper.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to International Patent Application No. PCT/US2019/30114 filed on May 1, 2019.

TECHNICAL FIELD

The present disclosure relates generally to electric propulsion devices, and more specifically to an electric propulsion system utilizing a heaterless dispenser cathode.

BACKGROUND

Satellites, and other small electrical units utilized in space based applications typically include on board propulsion systems in order to achieve precise control of the position and orientation of the units. Traditionally, such systems utilized chemical propulsion, as chemical propulsion provides large amounts of thrust. However, chemical propulsion systems require a large propellant mass, a high temperature and pressure, and consume potentially dangerous or difficult to handle propellants.

One alternative propulsion system is an electric propulsion system. Electric propulsion systems have a high exhaust velocity and fuel efficiency, and are generally divided into three categories: Electrothermal, Electrostatic, and Electromagnetic. A central part of electric propulsion systems is the utilization a cathode to generate electrons, which are used to ignite the discharge. Multiple different methods can be used to ignite a cathode discharge in a vacuum including gas injection, high voltage breakdown, mechanical actuators to create drawn arcs, and detonation of fuse wire.

SUMMARY OF THE INVENTION

In one exemplary embodiment a circuit for igniting and sustaining an electron discharge includes an ignitor circuit including a high voltage transformer, a switch connected in series between a primary of the transformer and a DC source return, the switch configured to receive a driving signal, a reset circuit connected in parallel to the primary of the high voltage transformer, a first rectifier connected in series between a secondary of the high voltage transformer and a keeper, a terminal of the secondary of transformer connected to a cathode, and a sustaining circuit having a current source having a return connected to a cathode, and a second rectifier connected in series between the current source and the keeper.

Another example of the above described circuit for igniting and sustaining an electron discharge further includes a plurality of power sources connected to at least one input power connection, the at least one power input connection providing input power to each of the ignitor circuit and the sustaining circuit.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the high voltage transformer is a high ratio transformer connected to a DC power source through the at least one power input.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the high ratio transformer has a winding ratio of at least 10 to 1.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the winding ratio is at least 100:1.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the winding ratio is approximately 60:1.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the circuit characterized by a lack of an output capacitor.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the reset circuit is a linear arranged diode and Zener diode.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the switch connected in series between a primary of the transformer and a DC source return switch is controlled via a controller.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the switch is a MOSFET (metal oxide semi-conductor field effect transistor).

In another example of any of the above described circuits for igniting and sustaining an electron discharge when the switch is control closed, and the DC source is applied across a primary of the transformer to produce a high voltage on a secondary of the transformer.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the current source comprises an isolated converter and an inductor.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the current source is a buck based converter.

In another example of any of the above described circuits for igniting and sustaining an electron discharge the sustaining circuit further comprises a DC load connecting an input of the second rectifier to a DC return of the isolated converter.

These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an electric thruster for a satellite.

FIG. 2 schematically illustrates an igniter circuit for utilization in the electric thruster of FIG. 1.

FIG. 3 schematically illustrates an exemplary core reset circuit.

DETAILED DESCRIPTION

Dispenser cathodes utilize a heater to warm up the dispenser cathode in order to allow the insert in the dispenser cathode to produce thermionic electrons. The thermionic electrons interact with low pressure gas in the dispenser cathode to produce ions and free electrons that make up the cathode discharge. The heaters are physical components that partially surround the cathode and convert stored electrical energy to heat energy. The presence of heaters increases the weight of the electrical unit, as well as increasing the electrical load drawn by the unit, but most significantly the heater is a reliability risk and is difficult to produce.

Dispenser cathodes are normally enclosed in another electrode referred to as a keeper. A major function of the keeper electrode is to facilitate turning on the cathode discharge and sustaining operation by providing an path to attract electrons when no other paths are available.

FIG. 1 schematically illustrates an exemplary electric propulsion system 10 for a satellite, or other space fairing electrical unit. The electric propulsion system 10 includes a power processing unit 20 connected to multiple power supplies 30. Within the power processing unit 20 is an igniter circuit 22. The circuit 22 for igniting and sustaining a cathode generates two power outputs, one of which is provided to a cathode 42 and the other of which is provided to an keeper 44. The cathode 42 and the keeper 44 are contained within a thruster 40. The thruster 40 is fluidly connected to a gas supply 50.

While illustrated in the example of FIG. 1 as including a single igniter circuit 22 and a single thruster 50, it is appreciated that the igniter circuit 22, and the thruster 50 could be duplicated as needed to provide any number of additional thrusters required for a given space fairing electric unit.

The circuit 22 for igniting and sustaining a cathode is configured to receive power from the power supplies 30 through the power processing unit 20, and to provide the processed power to the cathode 42 and the keeper 44. In order for the thruster 40 to ignite, and run the heaterless cathode 42, two electrical conditions are required from the outputs of the igniter circuit 22. First, the output provided to the igniter 44 must be a high enough voltage to break down the low pressure gas in the keeper 42. Second, the current from the circuit 22 for igniting and sustaining a cathode must be limited to a low enough magnitude that damaging vacuum arcs are prevented from occurring.

In ground based test systems (i.e. in laboratory conditions), this can be achieved using a 1500 V supply that is current limited to between 0.15 A and 0.3 A with an additional 1KΩ resistor in sequence. However, to achieve these conditions the circuitry used is bulky and expensive. Further, exactly matching the laboratory systems in a satellite would require too much weight to be implemented in a practical construction.

In order to achieve these conditions in a smaller package, the electric propulsion system 10 provides separate outputs from the power supply 22 to the keeper 44 and the cathode 42. With continued reference to FIG. 1, FIG. 2 schematically illustrates the igniter circuit 22 within the power processing unit 20 in greater detail.

The circuit 22 for igniting and sustaining a cathode is connected to the power inputs 30 via a processed power input 102, and can generally be divided into two circuits, an igniter circuit 101 and a sustaining circuit 103. Depending on the condition of the power provided from the inputs 30, the power received at the power input 102 can either be processed by additional circuitry within the power processing unit 20 or provided directly to the input 102 from the power supplies 30. Once received at the input 102, the power is provided to a high ratio transformer 110, a core reset circuit 114 and current source 105 including an isolated converter 130 and an inductor 140.

The high ratio transformer 110 provides a positive output to an output rectifier 120, which in turn provides the positive output to the keeper 44 via a power output 104. The negative output of the high ratio transformer 110 is provided to a second output 106 which outputs power to the cathode 42. The transformer 110 omits output capacitance and allows the transformer 110 to supply a high voltage without storing any energy. In some examples the winding ratio required of the high ratio transformer is on greater than 10:1.

The core reset circuit 114 is, in one example, a linear arranged diode 204 and Zener diode 202. This example is illustrated in FIG. 3. The core rest circuit 114 is connected in parallel to the transformer 110, with the high voltage side of the core reset circuit 114 being connected to a high voltage input of the transformer 110, and the low voltage side of the core reset circuit 114 being connected to a low voltage input of the transformer 110. The low voltage side of the core reset circuit 114 is connected to the non-powered leg of the transformer 110. The non-powered leg of the transformer 110 also connects to switch 112 that will connect the non-powered leg to neutral 108 when the switch 112 is closed, and disconnect the transformer when the switch is open. When the switch 112 is closed the diode 204 in the core reset circuit 114 blocks current through the core reset circuit 114. When the switch 112 is opened the diode 204 in the core reset circuit 114 allows magnetizing current through the core reset circuit 114 and the voltage drop across the Zener diode 202 is impressed on the transformer 110 primary providing the voltage to bring the flux in the transform 110 to a reset state.

During operation, an exterior control circuit (contained within the power processing unit 20, or elsewhere in the electric unit) provides pulses to the switch 112. The pulses effectively connect the bus voltage from the power input 102 across the primary winding of the transformer 110 to produce a high voltage on the secondary winding of the transformer 110. When the switch 112 is pulsed off, the parasitic capacitance in the cabling to the igniter 44 and the series impedance in the transformer 110 and switches 112 maintain the required high voltage, with some droop until the next on cycle of the switch 112. The transformer core and turn count is sized to prevent saturation of the core during the duration of the switch 112 being closed, and the sizing can be achieved according to any known methodology. In a practical implementation, during an ignition pulse train, the voltage provided to the igniter 44 is not flat (i.e., not constant) since there is a ramp up time when the switch 112 is on and a decay in the voltage during the off time of the switch. The general form of the resultant voltage provided to the igniter is a saw tooth pattern offset by a DC pulse.

The time period between the pulses of the switch 112 and the core reset circuit 114 provides the duration and voltage reset to the core of the transformer 110 and brings the transformer 110 back to flux zero. With this configuration, the voltage can be kept high indefinitely until a current is detected in the cathode, or pulses can be output in periodic groups to produce a pulse train to the igniter 44, depending on the specific needs of a given electric thruster 40.

If the turn on duration of the switch 112 is held constant, the magnitude of the ignition voltage is dependent on the input voltage to the igniter 44. Doing this can change the required ignition voltage by up to 50% for a typical input. The variability in the ignition pulse voltage caused by the input voltage is reduced in some examples by making the ignition pulse width dependent on the ignition voltage. This is achieved via any conventional digital or analog control system.

To provide the sustaining current through the Keeper 44 to the cathode 42 after discharge has started, an isolated converter 130 provides the requisite voltage and amperage. By way of example, the isolated converter 130 can be configured to provide anywhere 20 to 400 V at 0.15 to 0.3 A, and can be a push pull converter, or any other type of switch mode converter. In alternative examples, any other current magnitude can be achieved using a similar configuration with appropriate scaling.

In order to omit cathode heaters entirely, the cathode 42 is a self-heating cathode. The self-heating cathode converts a portion of the received electrical current to heat energy which then raises the temperature of the cathode. In order to ensure that a continuous current is received at the cathode 42, and thus the self-heating function is maintained throughout operation, an inductor 140 connects the output of the isolated converter 130 to a second rectifier 122. The second rectifier 122 is connected to the cathode output 106, and is a diode in the illustrated example.

The cathode 42 discharge sets the voltage to an effective DC voltage level while feedback of the current is used to control the current magnitude supplied to the cathode 42. The current source provides the DC current before the ignition of the cathode 42. In order to ensure this is achieved, the exemplary implementation of FIG. 2 includes a DC load 150. The DC load 150 can be a switched resistance, a string of Zener diodes, or any similar device that the discharge supply can be pre-charged into. The voltage of the load 150 is set to a higher level than the expected discharge so that when the cathode discharge is ignited, current naturally diverts from the load 150 to the cathode discharge. In order to prevent the discharge supply and load from shunting all the energy, the discharge and load 150 are connected to the igniter 44 through a high voltage diode 122. The high voltage diode 122 is sized to withstand a full magnitude of the ignition pulse.

It is further understood that any of the above described concepts can be used alone or in combination with any or all of the other above described concepts. Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention. 

1. A circuit for igniting and sustaining an electron discharge, comprising: an ignitor circuit including: a high voltage transformer; a switch connected in series between a primary of the transformer and a DC source return, the switch configured to receive a driving signal; a reset circuit connected in parallel to the primary of the high voltage transformer; a first rectifier connected in series between a secondary of the high voltage transformer and a keeper; a terminal of the secondary of transformer connected to a cathode; and a sustaining circuit having a current source having a return connected to a cathode; and a second rectifier connected in series between the current source and the keeper.
 2. The circuit of claim 1, further comprising a plurality of power sources connected to at least one input power connection, the at least one power input connection providing input power to each of the ignitor circuit and the sustaining circuit.
 3. The circuit of claim 2, wherein the high voltage transformer is a high ratio transformer connected to a DC power source through the at least one power input.
 4. The circuit of claim 3, wherein the high ratio transformer has a winding ratio of at least 10 to
 1. 5. The circuit of claim 4, wherein the winding ratio is at least 100:1.
 6. The circuit of claim 4, wherein the winding ratio is approximately 60:1.
 7. The circuit of claim 1, wherein the circuit characterized by a lack of an output capacitor.
 8. The circuit of claim 1, wherein the reset circuit is a linear arranged diode and Zener diode.
 9. The circuit of claim 1, wherein the switch connected in series between a primary of the transformer and a DC source return switch is controlled via a controller.
 10. The circuit of claim 9, wherein the switch is a MOSFET (metal oxide semi-conductor field effect transistor).
 11. The circuit of claim 1, wherein when the switch is control closed, and the DC source is applied across a primary of the transformer to produce a high voltage on a secondary of the transformer.
 12. The circuit of claim 1, wherein the current source comprises an isolated converter and an inductor.
 13. The circuit of claim 12, wherein the current source is a buck based converter.
 14. The circuit of claim 12, wherein the sustaining circuit further comprises a DC load connecting an input of the second rectifier to a DC return of the isolated converter. 